A radar sensor is described in PCT International Publication No. WO 97/02496. This document describes a monostatic FMCW radar sensor for a vehicle and for the detection of objects, in which at least one antenna feed is configured, in conjunction with a dielectric lens, both for transmission and for reception of a corresponding echo signal. According to one exemplary embodiment, this radar sensor has three antenna feeds which are each used for transmission and reception. For this, each of the three antenna feeds is connected via a transmission/reception splitter to downstream transmission and reception circuits. The transmission/reception splitter is implemented in the form of a ring coupler. This radar sensor as described is thus a triple-beam radar sensor which is suitable for determining an angular position of detected radar targets.
In the course of development, the following difficulty has arisen with a radar sensor of this kind: In order to achieve the necessary angular resolution required in order to assign detected objects to individual travel lanes, the antenna lobes which belong to the individual antenna feeds must be comparatively narrow. This has the disadvantage, however, that an object, for example a vehicle traveling ahead, which is located in an adjacent lane at a comparatively short distance in front of the vehicle equipped with a radar sensor of this kind can be detected only poorly or not at all. For example, with a radar sensor which possesses an observable angular region of +/-5 degrees, a vehicle traveling in an adjacent parallel lane and with a lateral clearance of 1 m will be detected only at a distance of 15 m. It is thus desirable to enlarge the observable angular region in the close-in range of a radar sensor of this kind. A variety of possibilities may be imagined for doing this, but several of these have associated disadvantages. For example, a widening of the observable angular region by way of a widening of the antenna lobes or a spreading of the antenna lobes would degrade the achievable angular resolution and reduce the detection range. The use of further, additional antenna lobes, on the other hand, results in elevated costs due to the additional signal processing circuits associated therewith. The same applies to mechanical or electronic slewing of the antenna lobes which are present.
A further possibility is to use a corrected ("shaped") lens system, with which, by means of specific shaping of the lens surfaces, a largely configurable antenna diagram can be generated. The disadvantage, however, is that such lenses are much thicker than the usual antenna lenses, and accordingly exhibit higher weight and greater losses. In addition, exact shaping is possible only for a single antenna feed, and degradations then occur in the other antenna lobes of a multiple-beam radar system. A lens system of this kind is moreover highly sensitive to mechanical tolerances, in particular, when a corresponding radar sensor is installed on a motor vehicle.
The stated problem of additionally detecting vehicles traveling alongside in the close-in range in fact becomes even worse on one side if a corresponding radar sensor cannot be installed in the center of the vehicle with respect to the vehicle's longitudinal axis. If a radar sensor of this kind is installed, for example, below the right front headlight of a vehicle, the "blind" area on the left side of the vehicle becomes larger.